Method of and apparatus for protecting sheet glass during the thermal treatment thereof



INVENTOR EMILE PLUMA '7' ATTORNEY E. PLUMAT Filed May 16, 1966 Q, Q. Q

\v vv July 23, 1968 METHOD OF AND APPARATUS FOR PROTECTING SHEET GLASSDURING THE THERMAL TREATMENT THEREOF United States Patent 3,393,987METHOD OF AND APPARATUS FOR PROTECTING SHEET GLASS DURING THE THERMALTREAT- MENT THEREOF Emile Plumat, Gilly, Belgium, assiguor to Glaverbel,Brussels, Belgium, a Belgian company Continuation-impart of applicationSer. No. 286,302, June 7, 1963. This application May 16, 1966, Ser. No.550,559 Claims priority, application Belgium, July 27, 1962, 495,905,Patent 620,787 Claims. (Cl. 65-29) This application is acontinuation-impart of my original application Ser. No. 286,302, filedJune 7, 1963, now abandoned.

This invention relates to the manufacture of sheet glass and isparticularly concerned with the protection thereof while in the form ofa continuous ribbon, or as separate sheets, during the finishing thermaltreatment of the surfaces of such sheet glass.

It is known to thermally treat sheet glass by floating it on a metalbath. In accordance with certain of these processes, the glass ribbonformed by rolling the glass between two rollers is slid onto a metalbath which brings the glass to a temperature that insures a fire polishof the two faces of the glass ribbon or strip. In others of suchprocesses, cut glass sheets are deposited on a metal bath which has beenraised to a fairly high temperature. Because of their displacement onsuch a bath, the glass sheets are heated to a temperature close to thesoftening temperature. The heated sheet glass is then placed in a rapidcooling chamber in which a metal bath at low temperature and jets ofcold air ensure hardening of the sheet glass. These prior processeshowever, have very substantial disadvantages which render their practiceundesirable. One disadvantage is that the usual fusible metals which areemployed in such baths, such as tin, rapidly oxidize in an atmospherewhich is but slightly oxidizing, at the working temperatures.Consequently, unless special precautions are taken, the oxide particleswhich are formed at the bath surface may damage the surfaces of thesheet glass. Another disadvantage of such prior processes, is thetendency of the alkali ions contained in the glass, generally Na+, todiffuse into the molten metal. It had been previously thought that thesediffused alkali ions were replaced by metal ions, for instance Sn++++,but it is now believed that the small quantity of metal which sometimesadheres to the sur' face of the glass after it is cooled result from aphysical and not from an ionic adhesion.

In order to overcome the disadvantage first above discussed, the artpracticed the heat treating operation in either an absolutely neutral,or a reducing atmosphere, but such practice was found to be veryexpensive. The art then proposed to protect the metal against theoxidizing atmosphere by means of a cover constituted of a molten salt.The sheet glass was then slid between the two baths of molten materialsso that it could be heat treated without disturbing the protectionafforded by the molten salt bath to the lower metal bath from theoxidizing amosphere. This method however, gave rise to another drawback,namely, the tendency of the ions of the molten salt, such as Li+, K Rb+and Mg++, to diifuse into the glass, and the tendency of the alkali ionsof the glass to diffuse into the salt bath. As a result of thisphenomenon and the exchange of ions. between the glass and the metalbath the surfaces of the sheet glass and sometimes the mass of the glassare considerably modified to the detriment of the glass sheet. Forexample, it has been found that when a sodium sheet glass is broughtinto contact with LiCl for ten minutes at a temperature of 850 C., thelithium ions will penetrate to a depth of 1 mm. into the glass,replacing the sodium ions of the glass. The lithium ion is smaller (1.2Angstrom) than the sodium ion (1.9 Angstrom), but at that temperaturestresses are avoided because of the internal rearrangement of themolecules. However, during cooling when the strain point of the glass isapproached, the outer layer which is rich in lithium, will be subjectedto longitudinal compression because the coefiicient of expansion oflithium glass is lower than that of the sodium glass. As a consequenceof the tension gradient between the lithium glass and the sodium glass,the surface of the sheet glass chips or shells very easily. On the otherhand, if the same glass is subject to the action of LiCl at atemperature below the strain point of the glass, for example, 400 C., nostructural rearrangement of the glass can take place, thereby subjectingthe outer layer of the glass to such longitudinal compression because ofthe smaller lithium ions which have replaced the sodium ions in suchouter layer, that such outer layer will be microscopically cracked tosuch an extent that it will assume a milky white appearance. Such aglass will break at the slightest compressive stress or shock. If thesame sodium glass is subjected to a salt of a more voluminous cation,such as KCl, at approximately the two above mentioned temperatures aboveand below the strain point of the glass, there will result in the caseof the high temperature treatment a glass having an outer layer which isstretched so that it is under tension and consequently rendering theglass sheet brittle, and in the case of the low temperature treatment aglass having an outer layer under longitudinal compression of suchextent that the glass sheet may break spontaneously.

It would seem that a likely solution to the problem would lay inprotecting the sodium glass by means of a molten sodium salt because insuch case the sodium ion migrations from the glass into the salt bathand vise versa would compensate themselves, and the practical resultwould be that the glass would be damaged to a very little extent.However, it has been found that other considerations often require theuse of cations which are different from that of the glass, or ofmixtures of salts of several cations, such as, for example, thefusibility of the salt over the whole range of temperatures of thetreatment of the glass, for instance, 1100 C. to 500 C., the vaporpressure of the salt, the corrosion of the refractory materials of thefurnace by certain types of anions, etc.

It is the principal purpose of the instant invention to retain theadvantages of heat treating sheet glass between a metallic bath and asalt bath, without incurring any of the aforesaid disdvantages of suchtreatment as heretofore practiced.

A further object of the invention is to provide a practical method ofprotecting sheet glass being treated as aforesaid no matter what theconstituents of the glass may be, or the conditions under which suchtreatment is being practiced.

I have found that the aforesaid purposes may be accomplished by applyingacross the interfaces of the metallic bath, the sheet glass and the saltbath, an electrical potential which is dependent on the potentialdifference existing between such items as a result of the ion diffusionbarriers at the interfaces thereof, and which is such that it reduces tothe desired extent the emigration of ions at such interfaces. I havealso found that while the diffusion barrier between the glass and themetal bath is practically nil, in spite of the existence of a diffusionof alkali ions into the metal, there exists a substantial diffusionbarrier between the sheet glass and the salt bath. In view of thisasymmetric character of the phenomena it is not believed possible todevise any standards which may be applied with any confidence. As in thecase of every energy barrier however, this diffusion barrier can beexplained in terms of an activation energy or by a potential difference.In the case of a molten salt containing but a single ion, there needonly be taken into account the barrier between this ion and the glass.In the case of salts containing several ions, however, the diffusionbarrier that should be taken into consideration is the one between themost electropositive ion and the glass. Thus, the salts of the followingshould be considered in the decreasing order in which they are arranged-Li Na K+, Rb+, Cs Mg and Mn The accompanying drawing illustrates by wayof example a longitudinal sectional view of a sheet glass furnaceconstructed for the practice of the invention.

In the drawing the reference numeral 7 indicates generally a tankcomposed of refractory material of the type usually employed in theconstruction of glass furnaces and having a base 8, side walls 9 and endwalls 10. Each of the end walls 10 is provided with a slot 11 equippedwith a fluid-tight joint 20 through which the glass sheet 12 enters andleaves the tank. The tank 7 is associated with a glass furnace of knownconstruction and the glass sheet may initially be drawn from the moltenglass bath 6 thereof vertically and then may be deflected horizontallyby a roll 5 in a known manner into the tank 7, as illustrated. Thus, theglass sheet is caused to enter the tank 7 directly from the furnace andbefore it has completely solidified.

The tank 7 contains a lower bath 13 of molten metal such as tin, lead,zinc and copper, such molten bath having a density higher than the glasssheet 12 floating thereon. The glass sheet 12 is sandwiched between themetal bath 13 and an upper salt bath 14 which may be of less densitythan the glass, or which may be of greater density than the glassbecause of the latters viscosity. For example, the salt bath may have adensity of 4 at 850 C. and the glass at that temperature a density of2.5 without the molten salt falling beneath the glass due to the lattersviscosity. Preferably the glass sheet is maintained in uniform contactwith the metal bath 13 by a series of rollers 17 engaging the upper faceof such sheet. It will thus be seen that shortly after the glass sheetissues from the drawing chamber it is inserted through one of the slots11 which are at the level of the interfaces of the two baths 13 and 14,and between such baths and advances across such superimposed baths tothe other slot 11 through which the glass sheet discharges.

Extending into the salt bath 14 are three vertically disposed rods 15which are connected by and electrical conductor 21 to an electricalinverter 19. Immersed in the metal bath 13 is a plate 16 of any suitableconfiguration dependent on the results to be attained and which isconnected by an electrical conductor 22 to the inverter 19. The rods 15and plate 16 are electrodes which are connected through the inverter 19and regulator apparatus, such as the potentiometer 28, to a voltagesource 18 which in the embodiment illustrated is such that theelectrodes 15 are positive with respect to the electrode 16. Byadjustment of the potentiometer 28 there is applied across theelectrodes 15 and 16 a DC. potential difference. In accordance with theinvention there is applied across the two baths a continuous DC.potential difference which is exactly opposed to that found existing bymeasurement as a result of the ion diffusion barrier or barriers presentbetween such electrodes, in order to nullify the tendency of the ions tomigrate from one material to the other. The DC. potential differenceexisting between the metal bath 13 and the salt bath 14 may be measuredby means of an electrorneter which measures voltage without consumptionof current, and employing the electrodes 15 and 16 in making suchmeasurement. After this measurement has been taken, there is applied tothe same electrodes a DC. potential difference which is opposed to thatdetermined by such measurement and which is between 0 and the valueobtained by such measurement.

Assuming for example, that it is desired to protect a sodium sheet glassinserted between a molten tin bath and a salt bath composed of a mixtureof LiCl, KCl and BaCl it will only be found necessary from what haspreviously been stated, to take into account the barrier of Li+ withrespect to the sodium glass, whatever the sodium content may be. Thus,since the barrier between the glass and the tin is zero and it is onlynecessary to take into account the most electropositive ion, namely,Li+, the barrier between the glass and the salt bath is 0.88 volt at 600C. Accordingly, the potentiometer 28 will be adjusted so that the saltbath will be brought to a potential of -0.88 volt with respect to thetin which is maintained at the potential 0, in order to avoid any damageto the surface of the glass. Under these conditions it will be found (1)that the lithium ion will not diffuse into the glass, (2) that the K+and Ba++ ions will not diffuse into the glass, (3) that the Na+ ions ofthe glass will not diffuse into the molten salt, and (4) that the Na+ ofthe glass will not diffuse into the molten tin. These phenomena aresurprising since they cannot be explained by considerations which areonly of an electrical nature. It would seem that these results areattained rather by a combination of electrical and geometricalphenomena. Thus, it may be that the lithium ion is electricallyprevented from penetrating into the glass, and that so long as thesodium ion of the glass is not expelled by the entrance therein of thelithium ions, the sodium ions are retained in the glass. However, thiswould not seem to explain the reason for the prevention of the diffusionof the sodium ion into the tin. Further, if the sodium glass weretreated in a salt bath composed of KNO AgCl and KCl at 650 C. one shouldstill only take into account the potassium-sodium glass barrier which isof the order of +0.35 volt, for it has been found that an opposingpotential difference of that order will not only prevent the diffusionof the K+ ion into the glass, but will also prevent the diffusion of theAg ion. This is surprising because the silver-sodium glass barrier is+0.60 volt, and it would therefore seem unlikely that the Ag+ ion of thesalt would be prevented from entering into the glass by a potentialdifference of only +0.35 volt applied to the salt.

Examples of metallic baths that may be used in the practice of theinvention are as follows:

Fusion Boiling temperature, 0. temperature, C.

Zinc and other combinations of tin, lead, zinc, copper, aluminum andsilver may advantageously be used as the metallic bath.

The salt bath may be composed of a molten salt containing one or more ofthe alkali or alkaline earth metals, and comprising cobalt, manganeseand the following examples of usable salt baths:

While I have heretofore more particularly referred to the treatment ofsodium glasses, it will be understood that any glasses whatsoevercontaining an alkali ion may be treated according to the inventionprovided the potential Fusion Boiling Density temperature, 0.temperature, C.

BaCl; 925 1, 560 3. 9 GaCIz 772 l, 600 2. 5 011201 422 1, 366 3. 53 LlBr545 1, 265 3. 51 L101, 613 1, 353 2. 5 M on 70s 1, 412 2. 5 KOL 776 1,500 2. 5 KNO 334 2. 5 80% AgC1+N e01. 420 1, 500 2. 42 NaOl c 801 1, 41a2. 5 10% 38.012 2% AgC 1% L1Br- 749 1, 528 2. 9 5% N aCL- 82% 08.010oizolz 422 2. 7 LlNOg 264 600 2. 38

Following are examples of the approximate D.C. potential differencesthat may be applied between the molten salt and the molten metal in thetreatment of sodium glass at the indicated temperatures:

The above table of potential differences is suitable for any glasscontaining sodium even though such glass might also additionally containpotassium or rubidium, and the potential differences given thereinshould be applied prefera-bly substantially as indicated. Thus, should asodium glass be treated between a molten lithium salt and any of theabove indicated molten metals at 600 C., it has been found that theideal potential dilference to be applied across the baths is 0. 88 volt.However, it has also been found that potential differences of between.88 volt and 0 will diminish the dilfusions and thus protect partiallythe surface of the glass, and that a potential difference of up to 1.08volts can be used without trouble. But it the potential difference isincreased above l.08 volts, it has been found that the Na+ ion Starts tobe extracted from the glass. Accordingly, while in the indicatedexample, .88 volt in the ideal potential difference to be employed,potential differences of approximately --0.2() volt above such ideal maybe utilized to accomplish the purposes of the invention. It will beremembered also that when a mixture of salts is used in the salt bath,the potential difference to be selected from the table thereof is theone indicated for the element which is the most electropositive. Alsoshould temperatures intermediate that indicated in the table be used,the potential difference selected should be proportional to thetemperature difference. Thus, in the case of the treatment of a sodiumglass, such as the following illustrative sodium glass designated No. 1,using a salt bath containing lithium and a temperature of either 1000"C., 800 C. or 650 C., the ideal potential differences used should beeither -1.27 vol-ts, -1.0 volt, or 0.92 volt, respectively.

Typical examples of sodium sheet glasses that have been treated inaccordance with the invention contained the following elements:

1 Traces.

difference is modified in accordance with the alkali ion containedtherein. Thus, assuming that the glass to be treated contains no sodium,and the salt bath contains no lithium, but contains potassium as itsmost electropositive element, all of the values in the above potentialdifference table diminished by 0.16 volt, the difference between the K+and Na+ ions, may be used in the treatment of such glass at thetemperatures and with salt baths containing the elements other thanlithium indicated in such table.

In the practice of the process the glass may be treated at any desiredtemperature and this temperature may vary along the path of the glasssheet through the bath provided the potential differences are varied totake care of such temperature variations. For example, if the differenceof temperature is such between two points in the path of travel of theglass, that it corresponds to a variation of approximately 0.2 volt withrespect to the value given in the aforesaid potential difference table,it Will be advisable to modify the DC. potential difference accordinglyto prevent the extraction of alkali ions from the glass. Thus, one mayarrange a single electrode of suitable length in the metallic bath asindicated in the drawing and arrange several electrodes at different DC.potential differences in the salt baths. Also, temperatures may 'beemployed for glasses in which the conditions thereof range from that ofa very fluid one, for instance, the above indicated glass No. l at l,300C. (viscosity=10 poises), to a solidified glass, for instance byapplying a temperature up to 500 C. (viscosity: 10 poises), passingthrough the softening point (T=762 C.) and the strain point (T=513 C.).

The protection afforded to the glass by the treatment is immediatelyeffective at all of the temperatures and is maintained for longdurations of time. Thus it has been found that portions of sheet glasshaving: the compositions of the aforesaid glasses Nos. 1 and 2, whichwere left between the molten metal and the molten salt (25% KCl, 50%BaCl and 25% MgCl within a temperature gradient of from 1000 C. to 650C., and with the application of adequate DC. potential differences for aperiod of more than 24 hours, were perfectly protected.

While I have hereinabove described and illustrated in the drawingsexamples of the manner in which the invention may be practiced, it will"be apparent to those skilled in the art that various changes may bemade therein without departing from the spirit of the invention or thescope of the appended claims.

I claim:

1. The method of protecting sheet glass subjected to thermal treatmentbetween a molten metal bath which is denser than the glass and a moltensalt bath of a density not greater than 4, characterized in that thereis measured with the sheet glass therebetween, the potential differenceexisting between a place in the molten metal and a place in the moltensalt, and then applying direct current electric power between suchplaces in the molten metal and the molten salt to create between suchplaces and across the sheet glass a direct current potential dilferenceopposed to and not substantially greater than the potential differencefound to be existing there'between and sufficient to substantiallyprevent the emigration of ions between the molten salt and the glass,and between the glass and the molten metal.

2. The method of claim 1, in which said existing potential differencebetween such places in the molten metal and the molten salt is measuredat a plurality of spaced portions of the sheet glass and if suchmeasured potential difference is variable at such portions, applyingopposing variable direct current potential differences at such portionsto prevent the ion emigration thereat.

3. The method of protecting sheet glass subjected to thermal treatmentbetween a molten metal bath which is denser than the glass and a moltensalt bath of a density not greater than 4, characterized in that thereis applied to the molten salt and between a place in the molten salt anda place in the molten metal, and across the glass sheet therebetweendirect current electric power capable of creating between such places apotential difference approximating that electric potential given in thefollowing table as a function of the temperature of a sodium glass forthe most electropositive element in the molten salt with which the glassis being treated at that temperature, such given potential beingdiminished by 0.16 if the glass is without sodium and treated in a saltbath which is without lithuim and contains potassium as the mostelectropositive element:

4. The method of protecting sheet glass thermally treated by floating itbetween the surfaces of a molten metal bath and a molten salt bath,which comprises inserting the formed glass sheet between twosuperimposed baths of immiscible materials, the lower bath beingconstituted of a molten material having a higher density than glass andcomposed of at least one metal selected from the group comprising tin,lead, zinc, copper, aluminum and silver, and the upper bath beingconstituted of a molten material having a density not greater than 4,and composed of a molten salt of one or more metals selected from thegroup comprising alkali or alkaline earth metals, cobalt, manganese,barium, calcium, copper, lithium, magnesium, potassium, silver, sodiumand chromium, and the halides thereof, and then applying between spacedplaces in the baths of molten metal and molten salt and across the glasssheet therebetween direct current electric power capable of creatingbetween such places a potential difference that will modify thepotential difference existing between such places in said baths as aresult of the barrier ion diffusion therebetween.

5. The method defined in claim 4, in which the applied direct currentelectric power is so selected that it creates a potential differencethat is substantially opposed to a potential difference constituted ofthe potential differences between such places created by the iondiffusion barriers between said lower and upper baths and the glasssheet.

6. The method defined in claim 4, in which the applied direct currentelectric power is variable to create a variable potential differencethat is variably applied to different portions of the glass sheet.

7. The combination in apparatus for protecting sheet glass beingsubjected to a thermal treatment between the surfaces of a molten metalbath and a molten salt bath, of a tank of heat refractory materialprovided on at least one side with a horizontal slot through which theglass r sheet is inserted into the tank, said tank being provided with apair of electrodes, one of such electrodes being located at a levelhigher than that of the horizontal slot and the other electrode beinglocated at a level below that of said slot, the tank containing up tothe level of the slot a lower bath of molten material which is denserthan the glass and constituted of at least one metal selected from thegroup comprising tin, lead, zinc, copper, aluminum and silver, and abovethe level of said slot an upper bath of molten material having a densitynot greater than 4, and constituted of a molten salt of one or moremetals selected from the group comprising alkali or alkaline earthmetals, cobalt, manganese, barium, calcium, copper, lithium, magnesium,potassium, silver, sodium and chromium, and the halides thereof, so thatsaid one electrode is situated in said upper bath and said otherelectrode is situated in said lower bath, and the glass sheet isinserted between the surfaces of such two baths and is exposed to anelectrical potential difference applied by the electrodes in such baths,and means for causing said electrodes to apply an electrical directcurrent potential difference that will modify the potential differenceexisting between said upper and lower baths as a result of the barrierion diffusion therebetween.

8. The combination defined in claim 7, in which said electrodes areconstructed and arranged to apply a potential difference acrosssubstantially the entire area of the glass in said tank.

9. The combination defined in claim 7, in which said electrodes areconstructed and arranged to apply a potential difference across a partonly of the area of the glass sheet in said tank.

10. The combination defined in claim 7, in which said electrodes areconstructed and arranged to apply the potential difference with variedintensity to different portions of the glass sheet in said tank.

References Cited UNITED STATES PATENTS 1,592,429 7/1926 Kraus -30 X2,198,733 4/1940 Lei'big et al. 65-30 X 2,754,559 7/1956 Fromson.3,218,220 11/1965 Weber 6530 3,242,060 3/ 1966 Le Cleric.

DONALL H. 'SYLVESTER, Primary Examiner.

F. W. MIGA, Assistant Examiner.

1. THE METHOD OF PROTECTING SHEET GLASS SUBJECTED TO THERMAL TREATMENTBETWEEN A MOLTEN METAL BATH WHICH IS DENSER THAN THE GLASS AND A MOLTENSALT BATH OF A DENSITY NOT GREATER THAN 4, CHARACTERIZED IN THAT THEREIS MEASURED WITH THE SHEET GLASS THEREBETWEEN, THE POTENTIAL DIFFERNCEEXISTING BETWEEN A PLACE IN THE MOLTEN METAL AND A PLACE IN THE MOLTENSALT, AND THEN APPLYING DIRECT CURRENT ELECTRIC POWER BETWEEN SUCHPLACES IN THE MOLTEN METAL AND THE MOLTEN SALT TO CREATE BETWEEN SUCHPLACES AND ACROSS THE SHEET GLASS A DIRECT CURRENT POTENTIAL DIFFENCEOPPOSED TO AND NOT SUBSTANTIALLY GREATER THAN THE POTENTIAL DIFFERENCEFOUND TO BE EXISTING THEREBETWEEN AND SUFFICIENT TO SUBSTANTIALLYPREVENT THE EMIGRATION OF IONS BETWEEN THE MOLTEN SALT AND THE GLASS,AND BETWEEN THE GLASS AND THE MOLTEN METAL.
 7. THE COMBINATION INAPPARATUS FOR PROTECTING SHEET GLASS BEING SUBJECTED TO A THERMALTREATMENT BETWEEN THE SURFACES OF A MOLTEN METAL BATH AND A MOLTEN SALTBATH, OF A TANK OF HEAT REFRACTORY MATERIAL PROVIDED ON AT LEAST ONESIDE WITH A HORIZONTAL SLOT THROUGH WHICH THE GLASS SHEET IS INSERTEDINTO THE TANK, SAID TANK BEING PROVIDED WITH A PAIR OF ELECTRODES, ONEOF SUCH ELECTRODES BEING LOCATED AT A LEVEL HIGHER THAN THAT OF THEHORIZONTAL SLOT AND THE OTHER ELECTRODE BEING LOCATED AT A LEVEL BELOWTHAT OF SAID SLOT, THE TANK CONTAINING UP TO THE LEVEL OF THE SLOT ALOWER BATH OF MOLTEN MATERIAL WHICH IS DENSER THAN THE GLASS ANDCONSTITUTED OF AT LEAST ONE METAL SELECTED FROM THE GROUP COMPRISINGTIN, LEAD, ZINC, COPPER, ALUMINUM AND SILVER, AND ABOVE THE LEVEL OFSAID SLOT AN UPPER BATH OF MOLTEN MATERIAL HAVING A DENSITY NOT GREATERTHAN 4, AND CONSTITUTED OF A MOLTEN SALT OF ONE OR MORE METALS SELECTEDFROM THE GROUP COMPRISING A ALKALI OR ALKALINE EARTH METALS, COBALT,MANGANESE, BARIUM, CALCIUM, COPPER, LITHIUM, MAGNESIUM, POTASSIUM,SILVER, SODIUM AND CHROMIUM, AND THE HALIDES THEREOF, SO THAT SAID ONEELECTRODE IS SITUATED IN SAID UPPER BATH AND SAID OTHER ELECTRODE ISSITUATED IN SAID LOWER BATH, AND THE GLASS SHEET IS INSERTED BETWEEN THESURFACES OF SUCH TWO BATHS AND IS EXPOSED TO AN ELECTRICAL POTENTIALDIFFERENCE APPLIED BY THE ELECTRODES IN SUCH BATHS, AND MEANS FORCAUSING SAID ELECTRODES TO APPLY AN ELECTRICAL DIRECT CURRENT POTENTIALDIFFERNCE THAT WILL MODIFY THE POTENTIAL DIFFERENCE EXISTING BETWEENSAID UPPER AND LOWER BATHS AS A RESULT OF THE BARRIER ION DIFFUSIONTHEREBETWEEN.